1. Field of the Invention
This invention generally relates to pulleys. More particularly, the present invention relates to an over-running clutch pulley which has particular application within an engine accessory system including an automotive alternator.
2. Description of the Prior Art
During operation of an engine, a belt drive system is sometimes used to power and operate the various accessory devices including, but not limited to, an alternator which provides electrical power to the vehicle. While several type of belt drive systems are in use, the system which is currently in favor is known as a serpentine drive system. Serpentine drive systems generally include a drive pulley connected to the crankshaft of the automobile""s internal combustion (I.C.) engine and a ribbed belt trained about the drive pulley. The belt is also trained about one or more driven pulleys which are in turn connected to the input shafts of the various accessories. An automatic belt tensioner is also provided to maintain the tension of the belt within the proper range. Most driven pulleys are provided in a one-piece design. These pulleys have no over-running capabilities, meaning that the pulley is rigidly mounted to rotate with the accessory input shaft.
When the input shaft of the accessory device is running at high speed (up to 22,000 rpm for an alternator), a significant amount of inertia is built up within the accessory device. As a result of the combined inertia and the lack of over-running capabilities, relative slipping between the pulley and the belt can occur if the belt decelerates too quickly. If significant slipping of the belt occurs, an audible squeal will be produced. Not only is a squealing belt annoying from an auditory standpoint, but it also undesirable from a mechanical standpoint since it produces undue wear on the belt itself.
In a typical driving situation, the accessory belt will experience instances of large deceleration, such as during in a 1-2 upshift during wide-open throttle acceleration. The situation is made even worse if the throttle is closed or xe2x80x9cbacked outxe2x80x9d immediately after the transmission has been shifted. In these situations, the belt decelerates very quickly while the pulley, with the high inertia from the accessory, keeps rotating very quickly in spite of the friction between the pulley and the belt.
Another major problem with the front end accessory drive is the torsional vibrations caused by the engine pulsations. Each cylinder in the internal combustion engine fires, a torsional pulsation is produced, causing a sinusoidal variation in the overall torque produced by the engine. In most operating conditions, the engine is rotating quickly enough that these torsional vibrations are smoothed out, and little if any vibration can be felt by the driver. The front end accessory drive is designed so that under most operating conditions, there is no resonant vibration in any part of the system, so the belt spans between the accessories usually operate smoothly with little if any vibration. However, resonant conditions can exist under certain accessory loads and engine speeds. Examples of this problem are when xe2x80x9cluggingxe2x80x9d or sub-idle speed is encountered in a manual transmission vehicle, or when high loads are experienced when the A/C and power steering are operating simultaneously at maximum output. During these conditions, one or more belt spans between the accessories can begin vibrating uncontrollably, producing noise, belt wear and the potential for the belt being thrown off the pulleys.
In attempting to cure the slipping and resonant vibration of the belt, and associated problems, various design proposals have been put forward. One proposed design includes the use of higher belt tensions. However, in these designs the belt tends to wear even more quickly. Additionally, the bearings associated with the input shaft of the accessory also have shown greater wear and a shortened useful life. Various other tensioner constructions have been proposed, but those tensioners have generally exhibited poor performance and were costly to implement.
Attempts to cure the belt slippage, vibration and squeal problem have also proceeded in another direction. While early driven pulleys were constructed in a one-piece design, newer pulley designs have been proposed where the driven pulley itself exhibits an xe2x80x9cover-runningxe2x80x9d capability. This allows the driven pulley to rotate relative to the input shaft of the accessory and therefore accommodate the inertia built up within the accessory.
U.S. Pat. No. 4,725,259 issued to Miyata discloses a construction where the driven pulley is mounted to the input shaft via a one-way clutch. The clutch only engages when the angular velocity of the pulley is accelerating. Otherwise, the clutch slips relative to the input shaft of the accessory. This design is intended to smooth out the recurring fluctuations of instantaneous velocity in the belt that is typical of an I.C. engine. The design smoothes out the corresponding recurrent instantaneous slipping of the belt relative to the driven pulley. No specific constructions for the Miyata one-way clutch are given in the disclosure of this patent.
U.S. Pat. Nos. 5,139,463 and 5,156,573, issued to Butzek et al., disclose alternator pulley constructions in which a coil spring is disposed in a space between a hub attached directly to the accessory input shaft and a pulley mounted for relative movement exteriorly of the hub.
In the ""463 patent, the two ends of the coil spring are respectively bent radially inward and radially outward so that one will engage the hub and the other will engage the pulley. In this patent, the spring is wound so that when a positive torque is applied from the belt to the pulley, the rotational movement of the pulley will be transferred to the input shaft of the accessory as a result of the spring xe2x80x9cwinding-upxe2x80x9d and the tangs engaging both the hub and pulley. Whenever negative torque is provided from the belt to the pulley, the spring enables the input shaft to disengage from the accessory and the hub to rotate relative to the pulley.
In the ""573 patent, the coil spring has one end bent radially outward. Additionally, the spring includes two sets of volutes, an intermediate set located between the other set and the bent end of the spring. The diameter of the hub and the inner diameter of the non-intermediate volutes are such that the volutes engage the hub when the pulley is being driven by the belt. When a negative torque is established between the alternator pulley and the input shaft, the volutes loosen with respect to the hub and allow slipping to occur. Importantly, the Butzek design of the intermediate volutes allows for a xe2x80x9cresilient rotational motionxe2x80x9d to ease the shock loading between the pulley and the hub but can cause fatigue problems in the spring.
U.S. Pat. No. 5,598,913 discloses a one way over-running clutch pulley in which a coil spring engages a composite cylindrical surface which is defined by both the sheave and hub. The spring is oriented so that torque is transferred from the sheave to the hub when the sheave is driving the hub. The spring allows slip to occur between these two components during deceleration of the drive belt""s recurring speed and torque fluctuations.
While the clutch pulleys of these patents may operate adequately in some respects, they have drawbacks in others. First, some prior art pulleys have experienced resonant vibration problems, especially during idle and sub-idle conditions. Another problem in over-running pulleys is hard lock-up. During engine idle, the large torque fluctuations which are characteristic of the internal combustion engine are smoothed out by an over-running pulley such as the pulley disclosed in the ""913 patent. During the brief periods of deceleration, the pulley over-runs. Because of its stiff nature in the drive direction, the hub would over (theoretically) rotate slower than the sheave""s input speed of the alternator and pulley (or sheave and hub) are equal. Such immediate and hard lock-up can cause its own noise, vibration, wear, and fatigue durability problems.
In view of the foregoing limitations and shortcomings of the prior art devices, as well as other disadvantages not specifically mentioned above, it should be apparent that there still exists a need for an improved one-way, over-running clutch pulley.
It is therefore a primary object of this invention to fulfill that need by providing a one-way over-running clutch pulley which overcomes the limitations and shortcomings of the prior art. Such a clutch pulley would find particular applicability with the accessory drive system of an automobile where it would allow for relative slip or compliancy between the pulley sheave and hub at the onset of lock-up. It is also an object of the present invention to provide a one-way, over-running clutch pulley which accommodates large decelerations of the belt so as to reduce or eliminate the belt squeal and wear that normally occur as a result of the belt slipping relative to the pulley. With respect to the present invention, compliance is built into the pulley to provide extra dampening to move the resonant condition output of the speed range under which the vehicle could ever be expected to be operated.
An object of this invention is to provide an over-running clutch pulley which provides a gradual transition from an unlocked state to a locked state.
Another object of this invention is to provide a one-way, over-running clutch pulley incorporating a compression spring for providing the compliance in the clutch pulley.
Yet another object of the present invention is to provide an over-running clutch pulley of a compact, low cost and lightweight construction, yet which is of a durable construction.
Briefly described, these and other objects of the invention are accomplished according to the present invention by providing what is hereinafter referred to as a compression spring compliant, one-way over-running clutch pulley. The device is intended for mounting to a rotational input shaft of a device, such as the alternator of an automobile. While the clutch pulley of the present invention is being specifically discussed in connection with automotive vehicles, it will be easily seen that the present invention has general applicability in a wide range of situations, particularly where it is desirable to provide compliance between a rotational input member and an output member to eliminate belt squeal and resonant vibrations.
The clutch pulley of the present invention is engaged by a driving member, such as a belt, which is rotatably being driven. The clutch pulley has several main components including an inner pulley hub, an outer pulley sheave, a bearing and a coil spring.
The pulley hub includes an output hub through which a mounting bore is defined. The bore is sized and shaped to receive the input shaft of the driven device, such as an alternator in a vehicle accessory system. The input shaft of the driven device is mounted to the output hub so that relative rotational movement between the output hub and the input shaft is prevented. Accordingly, rotation of the output hub will cause rotation of the input shaft of the driven device, e.g. the alternator. The output hub includes projections or similar features which engage one end of series of compression springs. The other ends of the springs are engaged by projections or similar features on an input hub.
The input hub is mounted radially outboard of the output hub and can rotate relative thereto. An axial extension off of the input hub defines a first inner surface which is generally cylindrically configured and is generally coaxial with the bore of the output hub and the shaft of the accessory.
The clutch pulley also includes a pulley sheave having a first portion adapted to engage a driving member which causes rotation of the pulley sheave. A portion of the pulley sheave defines a second inner surface, which is also generally cylindrically configured. The second inner cylindrical surface has a diameter which is substantially the same as the diameter of the first inner cylindrical surface of the input hub. The first and second cylindrical surfaces are coaxial with one another and adjacently located so that they cooperate to define a common or composite inner cylindrical surface. This composite inner cylindrical surface also partially defines a spring receiving cavity.
The pulley sheave is mounted to the input hub by a support bearing which permits relative rotation between the two components . The support bearing is located between concentric portions of the pulley sheave and input hub.
Received in the spring cavity mentioned above is a wrap spring. The freestanding outer diameter of the wrap spring is slightly greater than the diameter defined by the composite inner cylindrical surface. As a result, the wrap spring is in an interference and frictional engagement with the composite inner cylindrical surface and retained by the radially outward contact normal force inherently exerted by the wrap spring. The winding of the wrap spring is oriented in a direction that compressively loads the wrap spring whenever the pulley sheave is positively driven or accelerated relative to the input hub. This action tends to unwind the wrap spring and would effectively increase its diameter if it were not restrained by the inner cylindrical surfaces of the sheave and input hub. Conversely, the diameter of the wrap spring effectively decreases when the pulley sheave is negatively driven or decelerated relative to the input hub.
The effective increase in the diameter of the wrap spring during compressive loading, positive driving and acceleration of the pulley sheave relative to the input hub causes the wrap spring to exert an increased radially outward contact normal force on the inner cylindrical surfaces of the sheave and input hub. The normal forces, which result in engagement of the wrap spring with inner cylindrical surfaces, increase exponentially along the spring helix reaching a maximum at the midpoint. Because the normal forces determine the frictional forces and therefore the torque load which can be carried by the present clutch pulley, the initial normal force caused by the interference fit between the wrap spring and composite inner cylindrical surfaces obviates the need for a tang on the wrap spring if a sufficient number of spring volutes engage both portions of the composite inner surface. With this increased normal force being applied to both the first and second inner cylindrical surfaces, these surfaces effectively become xe2x80x9clockedxe2x80x9d together by the wrap spring and torque is transferred from the input member (belt) to through the pulley sheave and input hub to the shaft of the driven device. Depending on the amplitude and frequency of these recurring speed and torque fluctuations , a resonant condition may be s et up in the drive belt, causing a flapping motion resulting in noise and excessive belt wear.
To eliminate the resonant condition, the present invention provides for a construction which allows a xe2x80x9csofterxe2x80x9d or more compliant coupling between the wrap spring and the surfaces, and therefore between the sheave and the input hub. In a preferred embodiment of the invention, compliancy is accomplished by the interaction of the input hub, the compression springs, and the output hub. During a non-locked condition, the compression springs are fully extended. As lock-up initiates, the wrap spring first exerts its increased radially outward normal force on the composite surface defined by the sheave and input hub. Because the input hub and output hub can rotate relative to one another, the input hub begins to rotate relative to the output hub. As it does the projections of the input hub move toward the projections of the output hub compressing the springs located therebetween. Depending on the torque being transmitted, the compression springs may be partially or substantially completely compressed. To prevent the compression springs from being over-compressed, the projections of the input hub, the output hub or both are provided with stops that engage the opposing projection or the stop of that projection. At engagement of the stop(s), the input and output hubs are hard locked in the driving direction.
As an alternative to the wrap spring construction mentioned above, an alternate embodiment could employ a roller ramp or sprag type one-way clutch mechanism.
By adding the compliant feature to the over-running pulley, the pulley becomes a torsional spring in the accessory drive system, affecting the frequency response characteristics of the total system.
When a deceleration or negative driving is experienced between the pulley sheave and the input hub, such as during a wide open throttle 1-2 upshift, the shaft of the driven device will not immediately respond to the deceleration because of the inertia built up within the driven device and the input hub will over-run the pulley sheave. This causes the wrap spring to unload and xe2x80x9cwind-upxe2x80x9d, effectively decreasing the diameter of the wrap spring. Even a very slight reduction in the effective diameter of the wrap spring results in a corresponding reduction in the normal force exerted by the wrap spring on the two inner cylindrical surfaces which is sufficient to xe2x80x9cunlockxe2x80x9d the first and second inner cylindrical surfaces from each other and permitted relative rotation between the two surfaces. The input hub can therefore rotate under the inertia of the driven device via the shaft while the pulley sheave can rotate under the influence of the input member or belt.
Relative rotation in this manner reduces or eliminates relative slipping between the belt and the pulley and, importantly, associated squeal and premature wear problems.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.